Migrate mw doxygen to zuul-cloner and Trusty

Job is no more bound to gallium by using the push-doc macro see doc
https://www.mediawiki.org/wiki/Continuous_integration/Documentation_generation

Ubuntu Trusty has doxygen 1.8.6 (bug 46771)
Migrate to Zuul cloner (bug 72837)
TARGET_BASEDIR is recognized by mwcore-docgen.sh slave script and used
to override the destination.

Use DOC_SUBPATH generated by Zuul which is the version extracted from
the branch or tag refspec. mwcore-docgen no more carry that logic since
https://gerrit.wikimedia.org/r/#/c/173295/

Bug: 46771
Bug: 72837
Change-Id: I1fd6d23917c2098dbdad37d9e718335616207f68